1. Field of the Invention
This invention relates to a method of forming a high density ceramic circuit substrate suitable for mounting semiconductor elements, such as ICs and LSIs, and more particularly to a method of manufacturing a ceramic circuit substrate involving formation of a via hole for interlayer connection of the circuit.
2. Description of the Prior Art
With the progress in computer design to reduce size and integration density of the circuits, circuit mounting substrates are more and more being scrutinized with respect to wiring patterns and increasingly becoming more multi-layered.
Currently, in order to obtain a circuit substrate of this type, a so-called resin substrate is employed, where the resin, such as epoxy, polyimide, etc., is impregnated into the glass-cloth.
But, recently, a ceramic substrate having certain excellent property characteristics is being considered as a replacement for the resin substrate and research is now occurring for obtaining effective improvement in and utilization of the ceramic substrate.
A ceramic substrate provides various properties for achieving a high density circuit pattern because it has little thermal aging variation, has high intensity for external stress and sustains high surface smoothness since the material itself is very stable. Therefore, bare semiconductor element chips can be directly connected and mounted, such as by the wire bonding method, on the ceramic substrate. This is unlike the conventional method where parts are mounted on the resin substrate, namely using a mounting method where the molded IC or LSI in ceramic packages are mounted on the substrate. Resultingly, there is an improved density of the elements to be mounted and increased scrutinization of the circuit patterns. Moreover, since a ceramic substrate employs an inter-layer connecting method where a via hole is provided, namely a hole bored on a layer substrate and then a metallic paste filled in the hole, it provides an a blind through hole that can be formed very easily and resultingly, the wiring density in the thickness direction of substrate can be increased. This is unlike the inter-layer connecting method where a through hole is bored on the multi-layered resin substrate, namely a through hole bored after stacking the layers for the multi-layered resin substrate and then conductive plating carried out for this through hole. A circuit structure utilizing the ceramic substrate is disclosed, for example, in "IBM Multichip Multilayer Ceramic modules for LSI chips-Design for Performance and density-" of "IEEE Transaction, 1980 Vol. CHMT3, No. 3, No. 1, March, 1980".
However, the ceramic circuit substrate of this type has disadvantages in that formation of the aforementioned via hole is troublesome and moreover it is impossible to expect highly accurate formation.
Namely, formation of a via hole usually requires two steps in the process, including the boring onto the substrate and filling of metallic paste into the hole.
The boring step is normally performed using a laser beam, but a long boring time is required for beam irradiation and beam deflection scanning: Moreover, the step of filling of the via hole with metallic paste is frequently not carried out accurately and firmly.